Microwave emission of sonoluminescing bubbles.

Kordomenos et al. have attempted to measure single bubble sonoluminescence (SBSL) emission in the microwave window of water in a band of frequencies ranging from 1.65 GHz to 2.35 GHz [Phys. Rev. E 59, 1781 (1999)]. The sensitivity of the experiment was such that signals greater than 1 nW would have been detected. We show here that this upper bound is compatible with the radiation processes that we think generate significant emission at optical frequencies, electron-neutral and electron-ion bremsstrahlung. In fact, we argue that, almost independently of the specific assumptions concerning the hydrodynamics or the nature of the radiative processes, SBSL intensities exceeding that upper bound can hardly be expected.